The present invention relates to an epitaxial wafer for fabricating a high-intensity infrared light-emitting diode which is employed in optical communications and spatial transmission using infrared radiation. The invention also relates to an infrared light-emitting device fabricated from the epitaxial wafer and to an apparatus employing the device.
Light-emitting devices (hereinafter referred to as LEDs) employing Ga1xe2x88x92xAlxAs (hereinafter abbreviated as GaAlAs) compound semiconductors have been widely used in a light source in a wavelength range from infrared to visible red light. Although an infrared LED is employed in optical communications and spatial transmission, there has been increasing demand for a high-intensity infrared LED of increased capacity for transmitting data and increased transmission distance.
As has conventionally been known, a GaAlAs LED having a double-hetero structure (hereinafter referred to as a DH structure) exhibits emitted-light intensity higher than that of a GaAlAs LED having a single-hetero structure, and emitted-light intensity is enhanced by means of removing a substrate.
In fabrication of an LED employing a substrate-removed-type structure (hereinafter referred to as a DDH structure), a typical DH structure; i.e., only three layers consisting of a p-type cladding layer, a p-type active layer, and an n-type cladding layer, is epitaxially grown and then a substrate is removed, to thereby reduce the thickness of a produced epitaxial wafer. Such an epitaxial wafer is difficult to handle during processing into a device. In addition, the distance from a bottom surface of the device to the pn junction decreases, and a paste for bonding the device to a conductor migrates through a side face of the device, to thereby disadvantageously short-circuit the pn junction. In order to avoid this problem, a fourth epitaxial layer is added to the DH structure so as to ensure the overall thickness of the substrate-removed and finished epitaxial wafer and the distance from a bottom surface of the device to the junction. This constitution is standard for a DDH structure. The fourth epitaxial layer is designed to have a band gap wider than that of an active layer, so as not to absorb emitted light from the active layer.
The fourth epitaxial layer is advantageously formed as an n-type layer on the side of an n-type cladding layer, in consideration of suppression of overall electric resistance of a device, since in a GaAlAs semiconductor electron mobility is 10 or more times hole mobility. Thus, an n-type layer has an electric resistance lower than that of a p-type layer when carrier concentration and Al compositional proportion in two layers are identical.
When an n-type layer is formed on the side of an n-type cladding layer so as to dispose a p-type cladding layer as an LED surface, two arrangements are possible. In one case, as shown in FIG. 1, an n-type GaAs substrate 1 is employed, and on the substrate, a first n-type GaAlAs layer 2, a second n-type GaAlAs layer 3, an n-type GaAlAs cladding layer 4, a p-type GaAlAs active layer 5, and a p-type GaAlAs cladding layer 6 are epitaxially grown. In the other case, as shown in FIG. 2, a p-type GaAs substrate 7 is employed, and on the substrate, a p-type GaAlAs layer 8, a p-type GaAlAs cladding layer 9, a p-type GaAlAs active layer 10, an n-type GaAlAs cladding layer 11, and a second n-type GaAlAs cladding layer 12 are epitaxially grown.
When epitaxial growth is carried out on a p-type GaAs substrate and a p-type cladding layer is grown first, the distance between the pn junction and the GaAs substrate is smaller than a similar distance provided when an n-type layer is initially grown on an n-type substrate. Thus, the former pn junction is formed at comparatively high temperature. As a result, dopants in an active layer diffuse into the n-type cladding layer, to thereby shift the position of the formed junction from a metallurgical interface to within the n-type cladding layer. Such a phenomenon predominantly occurs when Znxe2x80x94having low diffusion energyxe2x80x94is employed as a dopant in the active layer. Although this phenomenon is advantageously utilized for elevating output power of red LEDs produced from such an epitaxial wafer, the phenomenon is disadvantageous in view of increase in response speed of LEDs, and preferably the position of the junction interface is not deviated from a metallurgical interface. Alternatively, germanium (hereinafter referred to as Ge) may be employed as a dopant in the active layer, because Ge is less diffusive than Zn. However, when the active layer is doped with Ge at a high concentration so as to elevate data transmission speed, the diffusion phenomenon is not suppressed.
In liquid phase epitaxy, Te is also employed as an n-type dopant. The segregation coefficient of Te increases as temperature decreases. When epitaxial growth is initiated from a p-type substrate, the carrier concentration of the active layer becomes lower than that of a similar active layer provided when epitaxial growth is initiated from an n-type substrate. Thus, injection efficiency decreases, thereby lowering the response speed of an LED.
As described above, when a fourth layer is added to a DH structure, epitaxial growth layers are formed on an n-type substrate, and a fourth n-type layer is added to an n-type cladding layer in an advantageous manner.
The fourth epitaxial layer may comprise two or more layers, in view of facility for constituting epitaxial layers. Specifically, epitaxial growth must be initiated under high Al compositional proportion conditions so as to produce a thick fourth n-type layer. In this case, forward voltage (VF) of a produced device increases due to high Al compositional proportion. In order to prevent increase in VF, the fourth n-type layer is divided into two or more layers, thereby lowering the maximum Al compositional proportion.
The present inventors have conducted extensive studies on enhancement of intensity of light emitted from an infrared LED having the aforementioned DDH structure, and have developed an epitaxial wafer of the structure having the following characteristics.
Impurities in a p-type cladding layer particularly lower the intensity of light emitted from an LED. Among them, oxygen atoms are the most detrimental. In relation to this, the thickness of the p-type cladding layer has an optimum value.
In this case, when the carrier concentration of the p-type cladding layer is controlled to fall within a certain range, the intensity of emitted light increases further.
Also in this case, the intensity of emitted light increases when the thickness of an active layer decreases.
In an LED having such a structure, a p-type inversion layer is generated at the interface between a first n-type GaAlAs layer and a second n-type GaAlAs layer, thereby possibly causing a thyristor defect. Particularly, the intensity of emitted light decreases considerably when the active layer is thinned.
The thyristor defect is caused by drastic increase (spike-like profile) in acceptor impurity (carbon) concentration of the portion in the second n-type GaAlAs layer within 2 xcexcm of the growth-initiated interface. When the concentration is 1xc3x971017 cmxe2x88x923 or higher, occurrence of the thyristor defect becomes more likely.
When Ge is employed as a dopant of the p-type active layer in an LED of the aforementioned structure, the intensity of emitted light varies from one production lot to another, even when the LEDs are produced through an identical process. In the course of investigation of the reason for such variation, the inventors have found negative correlation between emitted light intensity and Ge concentration in an n-type cladding layer or a second n-type layer. The present invention has been accomplished on the basis of these findings.
Accordingly, the present invention is directed to the following:
[1] an epitaxial wafer for fabricating an infrared light-emitting device, which wafer is obtained by sequentially forming, on an n-type GaAs substrate, a first n-type Ga1xe2x88x92X1AlX1As layer (0 less than X1 less than 1); a second n-type Ga1xe2x88x92X2AlX2As layer (0 less than X2 less than 1); an n-type Ga1xe2x88x92X3AlX3As cladding layer (0 less than X3 less than 1); a p-type Ga1xe2x88x92X4AlX4As active layer (0 less than X4 less than 1) which has an emission wavelength of 850-900 nm; and a p-type Ga1xe2x88x92X5AlX5As cladding layer (0 less than X5 less than 1), through liquid phase epitaxy, and, subsequently, removing the n-type GaAs substrate, wherein the p-type Ga1xe2x88x92X5AlX5As cladding layer has a thickness of 5-30 xcexcm and the p-type GaAlAs cladding layer has an oxygen concentration of 3xc3x971016 atoms/cm3 or less;
[2] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1], wherein the p-type Ga1xe2x88x92X5AlX5As cladding layer has a carrier concentration of 1xc3x971017 cmxe2x88x923 to 1xc3x971018 cmxe2x88x923;
[3] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the p-type Ga1xe2x88x92X4AlX4As active layer has a thickness of 0.05-0.4 xcexcm;
[4] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the peak carbon concentration of the portion in the second n-type Ga1xe2x88x92X2AlX2As layer within 2 xcexcm of the interface between the second n-type Ga1xe2x88x92X2AlX2As layer and the first n-type Ga1xe2x88x92X1AlX1As layer is less than 1xc3x971017 atoms/cm3;
[5] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the p-type Ga1xe2x88x92X4AlX4As active layer contains germanium as a predominant dopant and the n-type Ga1xe2x88x92X3AlX3As cladding layer has a germanium concentration of 3xc3x971016 atoms/cm3 or less;
[6] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the second n-type Ga1xe2x88x92X2AlX2As layer has a germanium concentration of 3xc3x971016 atoms/cm3 or less;
[7] a light-emitting diode fabricated by use of an epitaxial wafer for fabricating an infrared light-emitting device as recited in any one of [1] to [6]; and
[8] an optical communications and spatial transmission apparatus employing a light-emitting diode as recited in [7].